In order to drive the main lift and propulsion rotor of a rotorcraft, in particular via a main power transmission gearbox, a first engine installation is known that implements a gas turbine referred to as a “linked-turbine” gas turbine.
A linked-turbine gas turbine is provided with a gas generator comprising in succession at least one compressor, a combustion chamber, and at least one expansion turbine, the compressor being mechanically linked to the expansion turbine via a main shaft.
It is possible to fit the compressor with a plurality of compression stages that may be axial and/or centrifugal, and it is possible to fit the expansion turbine with a plurality of expansion stages.
In operation, the gas turbine sucks in ambient air. This air is then compressed prior to being directed to the combustion chamber.
A fuel is injected under pressure into the combustion chamber and it is then ignited together with the compressed air.
The gas that results from the combustion is then taken to the expansion turbine where it is expanded. This gas then drives the expansion turbine in rotation about its axis of rotation.
It should be observed that the expansion turbine drives the compressor of the gas generator via the main shaft.
In addition, the main shaft is not used solely for linking the compressor to the expansion turbine. For linked-turbine gas turbines, the main shaft also constitutes the working shaft of the gas turbine and it is suitable for driving a helicopter transmission gearbox, for example.
Although effective, linked-turbine gas turbines suffer from a pumping phenomenon. When this phenomenon occurs, streams of ambient air separate from the blades of the compressor, thereby tending to allow compressed air to back into the compressor.
Consequently, the temperature in the combustion chamber increases greatly. The blades of the linked turbine are then melted almost immediately, thereby destroying the gas turbine.
That is why linked-turbine gas turbines are hardly used any more, at least for helicopter applications.
Linked-turbine gas turbines have thus been replaced in engine installations by higher performance gas turbines known as “free-turbine gas turbines”.
A free-turbine gas turbine has a gas generator, of the type described above, followed by a working turbine that is secured to the working shaft of the gas turbine. Unlike a linked-turbine gas turbine, the main shaft of the gas generator and the working shaft are distinct. Consequently, a free-turbine gas turbine is sometimes referred to as a “two-shaft gas turbine”.
The gas coming from the expansion turbine of the gas generator then sets the working turbine into rotation, which drives the working shaft of the gas turbine.
Free-turbine gas turbines enable a helicopter to be operated without major difficulty.
Nevertheless, the fuel consumption of a free-turbine gas turbine is relatively high.
In addition, the gas turbines of an engine installation generally operate at a medium speed so as to deliver a medium power, e.g. the so-called maximum continuous power for a helicopter. Nevertheless, the engine installation is sometimes called on to deliver higher power for a short period, such as maximum takeoff power for a helicopter.
Consequently, the gas turbines are overdimensioned overall, so that they are capable temporarily of delivering extra power.
Thus, for most of the time, the gas turbine is delivering power that is remote from its maximum power, thereby leading to increased fuel consumption. The minimum specific fuel consumption occurs on a gas turbine in the vicinity of its maximum power, and increases rapidly at lower powers.